It's because of default php versions on Debian:
Debian 10 - php 7.3
Debian 11 - php 7.4
Debian 12 - php 8.2

I'm still on Debian 10 - a new test-install on Debian 12 throws errors when logging in to kolab-webadmin. Didn't investigate further...

Thanks for this happy news :-)

This has now been merged into Kolab:16, though the package is still building, but should become available shortly.

Thx for the News!

Problem is with loading order of php modules. On Ubuntu the fix for me was:

mv /etc/php7.4/cli/conf.d/30-kolabformat.ini /etc/php7.4/cli/conf.d/32-kolabformat.ini
mv /etc/php7.4/apache2/conf.d/30-kolabformat.ini /etc/php7.4/apache2/conf.d/32-kolabformat.ini

We're working on a fix for packaging.

Any further Developments here?

Given the fact that 3 years passed prior to this change, that the KDE community had to create a fork of libkolab, and that openSUSE dropped the package due to lack of upstream maintenance, there is no point in keeping this task open.

Because jsTimezoneDetect is not very active project I decided to add a workaround on PHP side. Fixed in https://github.com/roundcube/roundcubemail/commit/36485dfc345ad724f08eaa7ea30c3cc88e48a00d.

Copenhagen is, in principle, available. The reason for the parent ticket to exist has largely been obsoleted. This sub-ticket, and its sub-tickets, can therefore be closed.

I have filed now a bug at the jstz library, because I guess that would be the cleanest solution.
If that ignores Etc timezones coming from the internationalization API from the browser, then it calculates a proper Olson timezone.

It does not depend on Windows Server version.
It does not depend on Firefox version either.

I got more details: the client is Firefox 56 running on Windows Server 2012 R2. The timezone setting for the user in Roundcube is "auto".

Done.